1. Field of the Invention
The invention relates notably to a method for increasing the capacity of a transmission system using parallel waveforms.
It can be applied to any system whose modulation is an OFDM type of modulation or an OFDM (MC-CDMA or other) type modulation.
2. Description of the Prior Art
The 802.11, 802.16 and HyperLAN/2 wireless networks use OFDM (Orthogonal Frequency Division Multiplexing) waveforms. FIG. 1 shows sub-carriers of a classic OFDM modem. These types of modulation entail the simultaneous sending of several symbols on orthogonal sub-carriers and they form part of parallel modems.
The utility of these OFDM modems is that demodulation can be done in a simple manner. In general, at transmission, a cyclical prefix (FIG. 2) is introduced in order to preserve the orthogonality of the sub-carriers at reception. At reception, this cyclical prefix is removed from the signal, and then a Fourier transform is carried out on the OFDM symbols. Should the length of the channel be smaller than the length of a cyclical prefix, the symbols may be demodulated without any inter-symbol interference, or interference between the sub-carriers after the channel has been estimated. Other forms of parallel modems exist, for example modems using filtered OFDM waveforms and MC-CDMA waveforms.
Classically, a multiple transmission context entails the use of receivers which, in a first step, process the spatial domain, a spatial filtering operation being performed in order to separate the users. Then, in a second step, a classic single-transmitter processing operation is performed. These techniques are known in the case of CDMA (Code Division Multiple Access) transmission.
FIG. 3 shows a structure used to increase the number of transmitters Ei sending simultaneously and thus to increase the capacity of the transmission system. In order to accurately demodulate the symbols transmitted, it is generally necessary to use at least as many reception antennas Ar as transmission antennas.
Should the transmitters share the same transmission systems, for example the local oscillators, the symbols sent are estimated, for example, by separately processing the different sub-carriers because the orthogonality between the sub-carriers is kept. FIG. 4 shows the preservation of orthogonality in a multiple-sending context for a sub-carrier.
In this case, it is possible to estimate the symbols transmitted by using joint demodulation techniques. On each sub-carrier n, the signal observed in the case of linear modulations is expressed by the following relationship:yn=Hnan+bn  (1)where Hn is the matrix Nc×Nu containing the coefficients of the propagation channel for the sub-carrier n, where Nc is the number of sensors and Nu is the number of users making simultaneous transmission. The Nu×1 vector an contains the Nu symbols of the sub-carrier n of the different users. Finally the Nc×1 vector bn contains the samples of the noise for the different reception centers for the sub-carrier n.
Using the model of the signal received from the equation (1), several detectors can be used to estimate the symbols sent. For example, the method uses techniques of joint frequency detection such as the MLSE (Maximum Likelihood Sequence Detection) technique, the MMSE (Minimum Mean Square Error) technique and the DFE (Decision Feedback Equalization) family of techniques. These receivers are classically used for CDMA (Code Division Multiple Access) transmission.
While these techniques perform well, they are not suited to more complex systems, for example, when the transmitters do not share the same oscillator. A frequency offset between the transmitters may then appear and compromise the orthogonality between the sub-carriers.